Electric motor control system



Feb. 17, 1948. F. E. CREVER 2,436,203

ELECTRIC MOTOR CONTROL SYSTEM Filed May 27, 1946 2 Sheets-Sheet 1 Inventor" FredericK E. Crever",

by w His Attor n'eg.

ELECTRIC MOTOR CONTROL SYSTEM Filed May 27, 1946 2 Sheets-Sheet 2 F g.Z.

l I I l O Inventor: Frederick ElCrever,

His Attorney- Patented Feb. 17, 1948 ELECTRIC MOTOR CONTROL SYSTEMFrederick E. Crever, Scotia, N. Y., assignor to General ElectricCompany, a corporation of New York Application May 27, 1946, Serial No.672,461

7 Claims. 1

This invention relates to control systems, more particularly to systemsfor controlling the operations of electric motors, and it has for anobject the provision of a simple, reliable, and improved control systemof this character.

Still more specifically the invention relates to systems for controllinga motor to operate in predetermined speed and positional relationshipswith respect to another motor or with respect to metal being rolled in amill.

In carrying the invention into effect in one form thereof, a firstSelsyn is driven by a suitable driving means and a second Selsyn isdriven by a motor which it is desired to operate in predetermined speedand positional relationships with respect to the driving means for thfirst Selsyn. Each of the Selsyns has a polyphase primary winding and apolyphase secondary winding. The primaries are supplied from a suitablepolyphase source. Means are provided for producing a voltage of whichthe magnitude is proportional to the resultant of the Voltages of twophases of the Selsyns which are angularly displaced when the Selsyns arein phase. Means are also provided for producing a similar voltage fromtwo other phases of the Selsyns which are angularly displaced when theSelsyns are in phase. These two voltages vary oppositely, i. e., oneincreases and the other decreases as the relative phase displacement ofthe two Selsyns varies. Means responsive to the difference of these twovoltages is provided for varying the speed of the motor to cause themotor to drive the second Selsyn in predetermined phase relationshipwith respect to said first Selsyn.

In illustrating the invention in one form thereof it is shown asembodied in a system for controlling the operation of a flying shear. Anobject of the invention is to control the shear in such a manner as tosynchronize the angular position of the shear blades with the end of thelength of steel which is being advanced toward the shear so that the cutwill be made at the proper point.

For a better and more complete understanding of the invention, referenceshould now be had to the following specification and to the accompanyingdrawing of which Fig. l is a simple, diagrammatical sketch of anembodiment of the invention, and Fig. 2' is a characteristic curve whichfacilitates an understanding of the invention.

Referring now to the drawing, a flying shear having blades I is drivenby an electric motor 2.

Preferably, the motor 2 is of the direct current type. It is suppliedfrom a suitable source such as the adjustable voltage generator 3 whichis driven at a speed that is preferably substantially constant, bysuitable means, such as an induction motor (not shown). A length ofsteel 4 is passed through the successive stands of a roughing milltoward the shear. Only the fifth, sixth, and eighth stands of theroughing mill are illustrated in the drawing. They are driven at asuitable speed by suitable driving meanssuch as electric motors. theeighth stand is illustrated.

It is desired that the shear shall crop the end of the piece apredetermined distance from the end. If the cut is made at a longerdistance, steel will be wasted, and if made at a shorter distance notall of the useless metal of the ragged end will be removed and anentirevlength will be wasted.

For the purpose of controlling the motor 2 to drive the shear in correctpositional relationship with respect to the end of the length of steel,a pair of Selsyns 6 and I are provided. The Selsyn 6 is provided with aprimar polyphase winding, having phases 6a, 6b and 6c, and with apolyphase secondary winding having phases 661, Be and 6f. Similarly, theSelsyn I is provided with a polyphase primary winding, having phases 1a,1b and 1c, and a polyphase secondary Winding having phases 1d, 76 andIf.

The primary windings of the Selsyns are illustrated as mounted on thestator members and the secondary windings as mounted on the rotormembers. However, the reverse arrangement may be employed if desired.

An electromagnetic contactor 8 is provided for the purpose of connectingthe primary windings of both Selsyns to a polyphase source ofalternating voltage which is represented by the three conductors 9. Therotor member of Selsyn 6 is mechanically coupled to the shaft of themotor 5 by means of an electromagnetic clutch Ill. The rotor member ofSelsyn 1 is directly connected to the shaft of the motor 2.

As shown, the adjustable voltage generator is provided with a fieldwinding 3a which is excited from a suitable exciter dynamo-electricmachine I I. This machine may be of any suitable type, such, forexample, as the compensated armature reaction excited type described andclaimed in Patent 2,227,992, Edwards and Alexanderson,

which patent is assigned to the assignee of this application.

Only the motor 5 for driving.

the speed of the motor 2 is derived from a potentiometer l3 which isconnected across the armature terminals of motor 2.

For the purpose of further controlling the speed I of the motor 2 inaccordance with the relative phase of the Selsyns 6 and I, an exciterdynamoelectric machine M is provided which is excited in accordance withthe difference of two voltages which are derived from the Selsyns.Although the exciter M may be of any suitable type, it is preferably anarmature reaction excited machine such as the machine ll. Its armatureis connected in circuit with the field winding Ha of machine H. Thiscircuit is traced from the negative terminal of tachometer generator l2,through field winding Ha to the negative terminal of the potentiometerl3, slider I3a, armature of dynamo-electric machine M to the positiveterminal of the tachometer generator l2. Both exciters H and M aredriven at a constant speed by suitable means such as an induction motor(not shown).

The machine M is provided with two control field windings Ma and Mbwhich act differentially or in opposition to each other to produce a netor resultant control efiect which is responsive to a difference in phaseof the Selsyns when they are out of phase but are rotating at the samespeed.

The control field winding Ma is energized in accordance with themagnitude of the vector resultant of the voltages of two of the phasesof the Selsyns 6 and 1 which are out of phase when the Selsyns are inphase. For example, this field winding is energized in accordance withthe magnitude of the vector resultant of the voltages of the phases 6eand 7d. The voltage of phase 66 lags the voltage of the phase 1d whenthe Selsyns are in phase. Similarly, the control field winding Mb isenergized in accordance with the magnitude of the resultant of thevoltages of the phases 6d and Fe. The voltage of phase 6d leads thevoltage of phase 'ie when the Selsyns are in phase.

As shown, the control field winding Ma is connected in series with thephases Be and 1d through a full wave rectifier l5, and the phases 6 andU which are connected together. Similarly, the control field winding Mbis connected in series with the phase windings 6d and 1e through a fullwave rectifier l6 and the phases Bi and if.

A flag switch I! is provided between the fifth and sixth stands of themill and is arranged to close its contacts l'ia and I'll) when the flagH is engaged by length of steel 4. I

An electromagnetic contactor i8 which is controlled by the contact l'l'bof the flag switch is provided for the purpose of interrupting theconnections between Selsyn 6 and the rectifiers l and It when the steelis not in contact with the flag. At this time it also serves to completethe connections between direct current source 19 and the primary phases6a and 6b and the secondary phases 3d and 5c of the Selsyn 6.

, rectifiers l5 and I6.

With the foregoing understanding of the elements and their organization,the operation of the system will be readily understood from thefollowing description.

The motor 2 drives the blades l of the shear at a speed whichcorresponds to the speed at which the eighth stand of the mill is drivenby the motor 5. When this speed relation-ship obtains, the differencebetween the voltage of the tachometer generator and the voltage tappedofi at the potentiometer l3 by the slider I341 is just sufficient toenergize the field winding Ila to maintain a balanced condition of thesystem. If the speed of the eighth stand is increased, the voltage ofthe tachometer generator will increase, thereby to increase-thedifference between the voltage of the tachometer generator l2 and thevoltage at the slider I3a. This increases the excitation of the fieldwinding Ha, thereby increasing the excitation of exciter l I whichresults in increasing the voltage of the generator 3 and the speed ofthe motor 2 until the balanced condition is restored.

Similarly, if the speed of the eighth stand is decreased, the differenceof the voltage at the;

slider I31; and the voltage of the tachometer generator is decreased,thereby decreasing the -ex-- rupting the driving connection between themotor, 5 and the rotor of Selsyn 6. Also, with the contacts llb open,the contactors 8 and I8 arede energized and open. In its open position,contactor 8 disconnects the primary windings of the Selsyns 8 and i fromthe alternating voltage source 9. The normally closed contacts ofcontactor l8 connect the phases 6a, 6b, 6d and 6c in series to the D.-C.source [9. rotor member of Selsyn 6 is rotated to and locked in apredetermined initial position. 7

When the length of steel 4 strikes the flag, the switch [1 closes itscontacts Na and lib. In closing, contacts lib complete energizingcircuits for-the contactors 8 and 18. In response to energization thecontactor 18 opens its normally closed contacts to interrupt the supplyof direct current to phases 6a, 6b, 6d and 6e and closes its normallyopen contacts, to complete the connections between the phases 6d and 6eand the Simultaneously, the con tacts Ila in closing energize theelectromagnetic clutch l 0, causing it to complete the drivingconnection between the motor 5 and the rotor of Selsyn B.

Assume that the phase rotation of the Selsyns is counter-clockwise.

syns 6 and l are in synchronism and inphase.

Under these conditions, the control field windings Ma and Mb will beequally and oppositely energized and thus will produce no controllingeiTect.

The polarity of the control fieldwinding Ma is such as to tend toincrease the excitation of the field winding Ila and thus to increasethe Consequently the I Assume further that when the clutch is engagedthe shear blades have the. correct position relative to the position ofthe end of the length of steel 4. Consequently the Selof'its twofieldwindingsisthe more strongly energized;

Assume now that the position of'the blades I begins-to lag the correctposition with respect to the'end-of'the length of steel 4. This causesthe Selsyn l to lag the Selsyn 6, thereby increasing the resultant ofthe voltages of-the phases 6e and 1d and decreasing theresultant of thevoltages of thephasesfid and 'le. Consequently the energization of thefield'winding- [4a is increased and that of the-field winding I427 isdecreased. This results in increasing the net excitation of the exciter'M in a direction to increase the excitation of field winding H and thusto increase the speed of the motor 2. As a result, the phase of theSelsynis advanced and likewise the blades l of the" shear are restoredto their correct position relative to the end of the length of steel.

Similarly, if the position of' the blades becomes advanced with respectto the end of the steel strip, the resultant of'the voltages of phases6e and Iddecreases and the resultant of the voltages of phases 6d and 1eincreases. This decreases the excitation of the field windin [4aand-increases the excitation of the field winding [4b, therebyincreasing the net excitation of the exciter M in a direction to buckthe excitation of the winding I la. This results in decreasing the speedof the motor 2 thereby restoring the Selsyns to synchronism andrestoring the correct position of the shear blades relative to theposition of the end of the length of steel.

If the shear blades are not in the correct. position relative to the endof the steel when the clutch is engaged, the Selsyns will not be insynchronism, Consequently the net excitation of the exciter M will havea polarity to increase or decrease the speed of the motor 2' as requiredto bring the Selsyns into synchronism and the blades into the correctposition relative to the end of the length of steel.

It is possible for. the field windings Ma and MD to have'equalenergizations when the Selsyns are operating exactly 180 degrees out ofphase.

However, this is an unstable-operating condition because the slightestdeparture of the Selsyns from this 180 degree position causes a furtherdeparture andthe shear will always synchronize about the zero phasedisplacement condition which is the conditionreferred to as in phase.

By choosingthe opposite polarity of the dynamo-electric machine I4, theout of phase or 180 degree point. can be made the stable point andthezero degree point, the unstable point.

This is illustrated in Fig. 2 by the characteristic curve the. ordinatesof which represent the difierence between the two rectified voltageswhich are supplied to the. control field windings Ida and. Mb of theamplidyne l4 and the abscissas of which represent the phase angle bywhich the secondary voltage of the Selsyn 6 leadsthe secondary voltageof theSelsyn I. If the secondary voltage of Selsyn 7 leads that ofSelsyn 6' by an angle between 180 and 360 degrees the amplidyne I4 isexcited with its voltage in a direction to subtract from the voltage ofthe tachometer I2 which causes the shear to slow down momentarily untilthe 180 degree position is reached at which the voltage of thedynamoelectric machine is such that its voltage adds to the voltage ofthe tachometer generator which causes the shear to speed up until the180 degree point is reached. If the voltages are in phase there is noexcitation of the dynamo-electric machine. l4; but this point isunstablebecause a slight departure. from this point causesa furtherdeparture and hence the shear will always synchronize at'the degreepoint.

Although in accordance with the provisions of the patent statutes-thisinvention isdescribed as embodied in concrete formandthe principlethereof has been explainedtogether with the best mode in which itis nowcontemplated applying that principle, it will be understood that thee1e-- ments shown and describedare merely illustrative and that theinvention is not limited'thereto since alterations and'modificationswillreadily suggest themselves to persons skilled in the art withoutdeparting from the true spirit of this in-- vention orfrom the scope ofthe annexed claims. What I claim as new and desireto secure by LettersPatent of the United States is:

1'. A control system comprising a driving means, a first Selsyn having arotor member con nected to said drivingmeans and a stator memher, apolyphase primary winding on one of said members anda secondarypolyphase winding on the other of said members, an electric motor,asecond Selsyn having a rotor member connected to said motor and astator member, a polyphase primary winding on one member of said secondSelsyn and a polyphase secondary winding on the other member, andameansresponsive to the -.dilrerence of two voltages for varying the speed ofsaid motor to causesaid motor to drive the rotor of said second Selsynin predetermined phase relationship with the rotor Jf said first Selsyn,said voltages changing oppositely in magnitude with a change in relativephase of said Selsyns and each being the vector resultant of thevoltages of two different phases of said secondary windings.

2. A control system comprising a driving means, a first Selsyn having arotor. member connected to said driving means and a stator member, apolyphase primary winding on one of said members and a secondarypolyphase winding on the other of said members, anelectric motor, asecond Selsyn having a rotor member connected to said motor and astatormember,

a polyphase primary winding on one member of said second Selsyn and apolyphase secondary winding on the other member, means responsive to thedifierence of two voltages for varying the speed of said motor to causesaid motor to drive the rotor of said second Selsyn in predeterminedphase relationship with the rotor of said first Selsyn, said voltageschanging oppositely in magnitude with a change in relative phase of saidSelsyns and each being the vector resultant of the voltages of twodifferent phases of said secondary windings, means for producing acontrol voltage corresponding to the speed of said driving means, meansfor producing a control voltage corresponding to the speed of saidmotor, and means responsive to the difference of said control voltagesfor varying the speed of said motor to drive said second Selsyn inpredetermined phase relationship with respect to said first Selsyn.

3. A control system comprising a driving means, a first Selsyn having arotor member connected to said driving means and a stator member, apolyphase primary winding on one of said members and a secondarypolyphase winding on the other of said members, an electric motor, asecond Selsyn having a rotor member connected to said motor and a statormember, a polyphase primary winding on one member of said second Selsynand a polyphase secondary winding on. the other;member, and a dynamo:electrici machine excitedjin response to, thB I1if,- ference of twovoltages for varying thespeed of; said motor to cause said motor todrivesaid secondv Selsyn in predetermined phase, relationship with saidfirst Selsyn; and voltages changing oppositely in magnitudewith respectto each other in response to a change in relative phase of said Selsynsand each being the vector resultant of the voltages of two difierentphases of said secondary windings.

4. A control system comprising 7 a driving means; a first Selsyn havinga rotor member connected to said driving means and a stator member, apolyphase primary winding on one of said members and a secondarypolyphase winding on the other of said members, an electric motor, asecond Selsyn having a rotor member connected to said motor and a statormember, a polyphase primary winding on one member of said second Selsynand a polyphase secondary winding on the other member, means forproducing a control voltage which is proportional in magnitude to theresultant of the voltages of two phases of said Selsyns which areangularly displaced when the Selsyns are in phase, means for producing asecond control voltage which is proportional in magnitude to theresultant of the voltages of two other phases of said Selsyns which areangularly displaced when the Selsyns are in phase, and a dynamo-electricmachine excited in response to the difierence of said control voltagesfor varying the speed of said motor to cause said motor to drive saidsecond Selsyn in predetermined phase relationship with said firstSelsyn. V

5. A control system comprising a driving means, a first Selsyn having arotor member connected to said driving means and a stator member, apolyphase primary winding on one of said members and a secondarypolyphase winding on the other of said members, an electric motor, asecond Selsyn having a rotor m mber connected to said motor and a statormember, a polyphase primary winding on one member of said second Selsynand a polyphase secondary winding on the other member, means forproducing a voltage proportional to the speed of said driving means,means for producing a'voltage proportional to the speed of said motor,a. first dynamo-electric machine excited in response to the dififerenceof said voltages for varying the speed of said motor to equalize thespeeds of said Selsyns, means for producing a control voltage which isproportional in magnitude to the resultant of the voltages of two phasesof said Selsyns which are angularly displaced when said Selsyns are inphase, means for producing a second control voltage which isproportional in magnitude to the resultant of the voltages of two otherphases of said Selsyns which are angularly displaced when said Selsynsare in phase, and a second dynamo-electric machine excited in responseto the difierence of said control Voltages for varying the speed of saidmotor to cause said motor to drive said second Selsyn in predeterminedphase relationship with said first Selsyn.

6. A control system comprising a driving mean a first Selsyn having arotor member connected to said driving means and a stator member, a

polyphase primary winding on one of saidmem bers and a secondarypolyphase windingpn the;

other of said 'mernbers, an electric motor, a second and apolyphasesecondary winding on the other member, a tachometer generatordrivenby'said driving means for producing a voltage proportional to thespeed thereof, a potentiometer cons nected across the armature of saidmotor for producing a voltage proportional to the speed ofsaid motor, afirst dynamo-electric machine having a field winding connected incircuit with said generator and a portion of said potentiometer-forvarying the speed of said motor to equalize the speeds of said Selsyns,means for producing-a control voltage proportional in magnitude, totheresultant of the voltages of two phases of said Selsyns which areangularly displaced when said- Selsyns are in phase, means for producinga second control voltage which is proportional in mag nitude to theresultant. of the voltages of two otherphases of said Selsyns which areangularly displaced when said Selsyns are in phase, and a seconddynamo-electric'machine having a fieldwinding excited by the diiferenceof said control voltages and having its armature connected in circuitwith said tachometer generator and said potentiometer for modifying theexcitation of said first dynamo-electric machine to vary the speed ofsaid motor to cause said motor to drive said second Selsyn inpredetermined phase relation ship with said first Selsyn. v

7. A control system comprising a, driving means, an electric motor, a.first, Selsyn having a stator member and a rotor member, apolyphas'eprima y winding on one of said members, a polyphase secondarywinding on the other, of said members means for completing andinterrupting a driving connection from said driving meansto said rotormember, means responsive to the interruption of said connectionforsupplying direct current to a portion of each of said windings tomove said rotor to a predetermined initial position, a second Selsynhaving a stator member and a rotor member connected to said motor, apolyphase primary winding on one of the members of said second Selsynand a polyphase secondary winding on the other of said members, meansfor connecting the primary winding of said second Selsyn to a source ofalternating voltage, switching means for interrupting said directcurrent, connecting the primary winding of said first Selsyn to saidalternating voltage source and completing said driving connections,means for producing a first voltage proportional in magnitude to theresultant of the voltages of two phases of said Selsyns which areangularly displaced when said Selsyns are in phase, means for producinga'second voltage proportional in magnitude to the resultant of thevoltages of two other phases of said Selsyns which are angularlydisplaced when said Selsyns are in phase, and means responsive to thedifierence of said first and second voltages for varying the speed ofsaid motor to cause said motor to drive said second Selsyn inpredetermined phase relationship with said first Selsyn.

FREDERICK E. CREVER.

